When the heart pumps blood into a limb, it enlarges. Ring-shaped electrodes around the limb can detect the resulting impedance change. This electrical impedance plethysmography has been used to measure vessel volume changes. But the blood resistivity also changes with velocity. So it is difficult to determine whether the observed impedance changes are due to volume changes or resistivity changes. We have developed a new technique for separating these two variables in the human. We place the limb in a container filled with saline that has the same resistivity as that of blood. Any changes then are from resistivity changes in the blood. We will exploit this technique by making measurements at many locations and in many subjects. We will compare these results with those predicted from blood resistivity changes measured on a physiologically correct flow rig. From these results we will create a model for the origin of electrical impedance plethysmography. This model should be valuable in the accurate noninvasive assessment of vascular changes that are present in arteriosclerosis.